Michael W. Sayers

A GENERIC MULTIBODY VEHICLE MODEL FOR SIMULATING HANDLING AND BRAKING

SUMMARY A vehicle system dynamics model is presented that captures the essential braking and handling behavior of an automobile with independent suspensions on a flat surface. The model, which has 18 degrees of freedom, is described in sufficient detail for an engineer to reproduce it with a multibody simulation program. Further, a stand-alone computer program based on the model has been put on the internet for interested readers to download and run. The model has a simple generic representation of suspension kinematics that can represent the behavior of most independent suspensions. The generic model is demonstrated using a previously published vehicle description (the IAVSD Iltis multibody benchmark) and compared with a detailed multibody model. Close agreement was found between the two models both for eigenvalues and for nonlinear time history responses.

DYNAMIC PAVEMENT/WHEEL LOADING FOR TRUCKS WITH TANDEM SUSPENSIONS

This paper presents measurements of the vertical loads imposed on the road by a heavy commercial vehicle with tandem suspensions, and also the vehicle ride accelerations induced by roughness. Measured characteristics of the vibrations are compared with those predicted using a two degree-of-freedom nonlinear vehicle model.

Crosswind Sensitivity of Passenger Cars and the Influence of Chassis and Aerodynamic Properties on Driver Preferences

SUMMARY Results of vehicle crosswind research involving both full-scale driver-vehicle tests and associated analyses are presented. The paper focuses on experimental crosswind testing of several different vehicle configurations and a group of seven drivers. A test procedure, which utilized wind-generating fans arranged in alternating directions to provide a crosswind “gauntlet”, is introduced and described. Driver preferences for certain basic chassis and aerodynamic properties are demonstrated and linked to elementary system responses measured during the crosswind gauntlet tests. Based on these experimental findings and confirming analytical results, a two-stage vehicle design process is then recommended for predicting and analyzing the crosswind sensitivity of a particular vehicle or new design.

SYMBOLIC QUASI-STATIC AND DYNAMIC ANALYSES OF COMPLEX AUTOMOBILE MODELS

SUMMARY This paper describes how quasi-static and dynamics analysis methods are combined to simulate of a “full-scale” passenger car model. It also describes a novel method for combining symbolic and numerical equation formulations to generate quasi-static analysis algorithms. A quasi-static model is first used to simulate three-dimensional suspension kinematics and compliance properties. The results from this analysis are then used to fit coefficients to polynomials that describe the suspension motions. These coefficients are then employed in a dynamics model that is formulated using the same polynomial equations. The method is illustrated for an automotive vehicle model, using a suspension representation with 11 degrees of freedom (DOF), and a full-vehicle model with 14 DOF. The symbolic multibody program AUTOSIM is used to derive the equations and optimize them for computer solution. The overall simulation is sufficiently detailed to captures the influences of bushing compliances, and yet is computatio...

ESTIMATION OF RIDEABILITY BY ANALYZING LONGITUDINAL ROAD PROFILE

A method for estimating pavement rideability by computing a statistic called ride number (RN) from measured longitudinal profiles of the pavement is presented. The computational algorithm was developed with two objectives in mind: (a) relevance, as established by correlation between RN and mean panel ratings obtained for profiled pavements, and (b) portability, as determined by the ability of different profiling systems to obtain comparable RN values for profiles taken on the same pavement. Experimental data show that the new algorithm is comparable to previously developed RN algorithms in terms of its correlation with panel ratings. The new algorithm is much more portable; comparable RN values can be obtained from different profilers and the quasistatic Dipstick device. However, measures from ultrasonic profilers are not as accurate, and in some cases these devices are not acceptable for measuring RN. Although the fundamental relationships between profile characteristics and the human perception of ridea...

Using simulation to learn about vehicle dynamics

Mathematical models of vehicles are used to learn about vehicle behaviour. Traditionally, computer simulation analyses have been limited to experts who have specific questions about the vehicle dynamic performance. However, less experienced engineers can also run mathematical models when simulation technology is combined with a modern graphical interface and a database of vehicle properties. This paper shows how engineers and others can use simulation to rapidly gain experience in vehicle dynamics. Fundamental concepts involving steer, braking, and throttle inputs are demonstrated in an easy-to-use simulation environment. The paper describes features in the user interface that support quick learning on the part of the user. It also lists model features that serve to emphasise the most significant vehicle properties.

Exact Equations for Tractrix Curves Associated with Vehicle Offtracking

Abstract Vehicle offtracking behavior at low speeds is closely approximated by a geometric entity called a tractrix. This paper presents differential equations for generalized coordinates of a planar multibody vehicle model based on tractrix behavior. The equations are exact, can be used with any type of input path, are valid for forward and backward movements, and are much simpler than previously published formulations used to compute transient offtracking. The differential equations can be integrated using conventional numerical integration algorithms to obtain plots of the low-speed tracking performance of articulated vehicles. The equations were formulated symbolically by a computer program used to analyze the kinematic and dynamic behavior of multibody systems. Example numerical results are plotted.

APPLICATIONS OF SYMBOLIC MANIPULATION FOR INVESTIGATING THE NONLINEAR DYNAMICS OF GROUND VEHICLES

SUMMARY Symbol manipulation tools can aid in the analysis of the nonlinear dynamics of ground vehicles in several ways. The equations of motion, derived automatically in symbolic form, can be numerically integrated to obtain a single simulated test. Alternatively, the nature of the overall nonlinear stability can be determined if the equations are derived in a certain form. Formulating equations for the stability analysis involves considerable algebraic manipulation, such that the analysis is practical only with computer aid. Recent improvements of computer hardware and recently developed computer algebra software now allow a level of automation that makes these analyses practical for general use by engineers. Applications from both rail and road vehicles are given in this paper as examples.

Investigation of the Influence of Various Braking Regulations on Accident-Avoidance Performance

Passenger car braking regulations promulgated by the United States and various European countries are analyzed to determine the incompatibilities in brake proportioning that derive from the performance requirements of the regulations. Two vehicles, one produced in the U.S. and the other in Europe, with extreme loading conditions (as derive from option selection and vehicle loading) were considered. The analysis shows that the European regulations require a larger forward bias in brake proportioning than FMVSS 105-75. Computer simulations were used to predict the differences in braking performance which result from proportioning the example vehicles to comply with the different regulations. Conditions of straight-line braking, braking in a turn, and three surface friction levels were considered. In general, both vehicles were capable of achieving shorter stopping distances when proportioned to meet FMVSS 105-75 than when proportioned to comply with the European regulations, but the differences were small and could be considered trivial when a car is driven by an ordinary driver. When proportioned to meet FMVSS 105-75, the rear wheels of the two study cars sometimes locked up first, a result that the European regulations are designed to prevent. Braking-in-a-turn test procedures (which have been discussed both in the U.S. and abroad) were also studied through computer simulations and found to be relatively insensitive to the fore-aft proportioning of brake torques.